Breaker selection according to loop impedence

A crucial topic in electrical engineering!

When selecting a breaker according to loop impedance, we need to consider the following factors:

1. Loop impedance: The total impedance of the circuit, including the resistance and inductive reactance of the conductors, the motor, and any other equipment connected to the circuit.
2. Fault current: The maximum current that can flow in the circuit during a fault, such as a short circuit or ground fault.
3. Breaker rating: The maximum current and voltage rating of the breaker, as well as its thermal and mechanical capabilities.

Here's a general guideline for selecting a breaker based on loop impedance:

Step 1: Calculate the loop impedance

• Measure or calculate the resistance (R) and inductive reactance (XL) of the conductors, motor, and other equipment connected to the circuit.
• Calculate the total loop impedance (Z) using the following formula:

Z = √(R^2 + (XL)^2)

Step 2: Determine the fault current

• Calculate the fault current (I_f) using the following formula:

I_f = V_f / Z

where V_f is the fault voltage (typically 1.732 times the line-to-line voltage for a 3-phase system).

Step 3: Select the breaker

• Choose a breaker with a rating that exceeds the fault current (I_f) calculated in Step 2.
• Consider the following factors when selecting the breaker:
  • Thermal rating: Ensure the breaker can handle the fault current without overheating.
  • Mechanical rating: Ensure the breaker can withstand the mechanical stress caused by the fault current.
  • Voltage rating: Ensure the breaker is rated for the same voltage as the circuit.
  • Current rating: Ensure the breaker is rated for the maximum current that can flow in the circuit.

Example

Suppose we have a 3-phase, 480V, 30A circuit with a loop impedance of 0.5 ohms. We want to select a breaker that can handle a fault current of 100A.

1. Calculate the loop impedance: Z = √(0.5^2 + (0.5^2)^2) = 0.707 ohms
2. Calculate the fault current: I_f = 1.732 x 480V / 0.707 ohms = 100A
3. Select the breaker: Choose a breaker with a rating of at least 100A, 480V, and a thermal and mechanical rating that can handle the fault current.

In summary, when selecting a breaker according to loop impedance, we need to consider the fault current and ensure the breaker is rated for the maximum current that can flow in the circuit, as well as its thermal and mechanical capabilities.